Conventionally, there have been various obstacle detection devices for vehicle which detect obstacles around a vehicle. Among them is one that self-diagnoses the detecting capability of the device itself without any inspecting device additionally installed (refer to Patent Document 1: JP-A-11-94946 (U.S. Pat. No. 6,147,637), for example). In detecting capability judgment processing by the obstacle detection device for vehicle disclosed in Patent Document 1, judgment is made as follows: for example, a vehicle ahead (or preceding vehicle) is driving away from a vehicle of interest. The device acquires a distance at which the device becomes incapable of detecting the distance to (lose track of) the vehicle ahead (sight end distance). If the average value of the sight end distances acquired by a predetermine number of times is not more than a predetermined distance, it is judged that the distance detecting capability has degraded.
When it is thereby judged that the device maintains the normal distance detecting capability, various types of control are performed. For example, following distance control (control wherein the distance between the vehicle of interest and the vehicle ahead is kept at a predetermined value) is performed. When it is judged that the device does not maintain the normal distance detecting capability, various measures are taken. For example, control, such as following distance control, is inhibited.
In conventional obstacle detection devices for vehicle, the above-mentioned detecting capability judgment processing is performed as follows: a sight end distance is acquired for the judgment of distance detecting capability where there is no influence of a blind spot. If there is a cut-in vehicle between a vehicle ahead to which the sight end distance is to be detected and the vehicle of interest, a problem arises. The vehicle ahead is hidden behind the cut-in vehicle and cannot be detected (creation of blind spot). The distance at which the vehicle ahead becomes undetectable can be erroneously judged as a sight end distance. To cope with this, the following measure is taken: Overlapping of the vehicle ahead and the cut-in vehicle is checked based on coordinate data indicating the past positional relation between the vehicles in the direction of the width of vehicle. If it is revealed from the result of the check that track of the vehicle ahead has been lost by the cut-in vehicle, detecting capability judgment based on the acquired sight end distance is inhibited.
However, this poses a problem. In the conventional detecting capability judgment processing, judgment is made based on coordinate data which indicates the positional relation between the vehicle ahead and the cut-in vehicle. In other words, the judgment processing is on the precondition that both the vehicle ahead and the cut-in vehicle have been detected. There is a case where a cut-in vehicle is actually present between a vehicle ahead and the vehicle of interest but reflected waves sufficient for distance detection cannot be obtained from the cut-in vehicle because of, for example, dirt on the rear part of the cut-in vehicle. In this case, the distance to the vehicle ahead which has been hidden behind the cut-in vehicle and undetectable is judged as the sight end distance, and the distance detecting capability is judged using this sight end distance.